Helminth parasites are endemic in many developing countries. The overall burden of disease due to these pathogens is high, with approximately 300 million infected people suffering from severe morbidity. Here we will focus on M2 macrophages, which play crucial host protective roles in helminth infections. Macrophages can adopt different activation states depending on context. Interferon-? in combination with TLR agonists promotes M1 (or classical) activation, whereas the cytokines IL-4 and IL-13 promote M2 (or alternative) activation. Differential functions in M1 and M2 macrophages are supported by distinct core metabolisms, with M1 cells committing to aerobic glycolysis, and M2 macrophages utilizing fatty acid oxidation (FAO) and mitochondrial oxidative phosphorylation. Indeed, FAO has been shown to be necessary for M2 activation. This realization has focused our attention on fatty acid metabolism, which since it is essential for M2 activation must, by definition, be an essential face of protection against helminth infections. Fatty acids for FAO are derived from the lysosomal lipolysis of triacylglycerols, which are sourced either from the exterior or through endogenous synthesis of fatty acids. Despite the fact that it is clear that FAO is essential for M2 activation the underlying reasons for the importance of this type of metabolism for M2 activation are unclear. We hypothesize that FAO serves as an efficient source of citrate for export into the cytosol where it can be used for processes that are critical for M2 activation such as synthesis of fatty acids for FAO and to act as ligands for PPARs, and the production of acetate for the acetylation of histones to permit the expression of M2 genes. In addition to their role in immunity to helminths, M2 macrophages play roles in wound healing, and in whole body metabolic homeostasis. Remarkably, helminth infection has been reported to be capable of mitigating the metabolic consequences of a high fat diet. We hypothesize that strong type 2 immunity induced by helminth infection is able to maintain metabolic homeostasis by broadly supporting M2 activation in tissues distal to the site of infection and will explore this in our third aim. The specific aims of this project are: 1. To establish whether fatty acid synthesis is necessary for M2 activation. 2. To assess the metabolic control of histone acetylation in M2 activation. 3. To determine whether type 2 immunity in helminth infection modulates metabolic homeostasis by broadly supporting M2 activation. We will use genetic loss of function and chemical inhibitor approaches to target key genes encoding enzymes, transcription factors and receptors involved in lipid metabolism to explore M2 activation in reductionist systems and during infection with helminth parasites. We will examine the outcome of alterations in lipid metabolism on infection with these organisms, and will examine the underlying basis for beneficial effects of helminth infections on metabolic disorders associated with obesity.